DC motor drive system for reducing start-up current

ABSTRACT

A system for driving a constant current drive type DC motor a which enables the supply of a low power to switching elements in a constant current drive circuit therein during, and in particular, at start-up of the motor. The DC motor drive system includes a DC motor, a unit for detecting a rotation of a rotor of the motor, a power supply unit for supplying a drive power to the motor, and a current drive circuit providing a constant current passing through a coil of the motor supplied with the drive power during a normal operation of the motor. The power supply unit sequentially increases a voltage therefrom in multi-steps in response to the increase of the rotation during a start-up operation. The power supply unit may change the voltage supplied therefrom so that a maximum value of the current is sequentially increased during the start-up.

BACKGROUND OF THE INVENTION

The present invention relates to a system for driving a DC motor, moreparticularly, to a system for driving a DC motor employed in, forexample, a large magnetic disc system for driving a spindle therein, andconnected to a constant current drive circuit.

FIG. 1 is a diagram of a prior art bi-polar direct current (DC) motordrive system employing a very bulky magnetic disc system (not shown). InFIG. 1, reference 1 denotes a three-phase brushless Hall-type DC motorincluding three exciting coils 2A, 2B, and 2C, and Hall-effect-typesensors 3A, 3B, and 3C, 4 denotes a circuit for synthesizing outputsSHG_(A) to SHG_(C) from the Hall sensors 3A, 3B and 3C, 5 denotes atiming control circuit, 6 denotes a phase exchange switching circuit, 7denotes a power supply (power source), and 8 denotes a constant currentdrive circuit having a phase switching function.

The phase exchange switching circuit 6 includes three power transistortype switches 6A to 6C. The constant current drive circuit 8 alsoincludes three constant current sources 8A to 8C each having at leastone power transistor.

A rotor (not shown) of the DC motor 1 is mechanically connected to aspindle (not shown) of the magnetic disc system, rotating a magneticdisc)s) (not shown) in response to the rotation of the spindle.

The rotation position of the rotor of the DC motor 1 is detected by theHall sensors 3A to 3C. The signals SHG_(A) to SHG_(C) output from theHall sensors 3A to 3C are synthesized at the signal synthesizing circuit4, resulting in a phase signal SPHASE. The timing control circuit 5generates timing signals ST_(A) to ST_(C) for energizing the powertransistor switches 6A to 6C and control signals SC_(A) to SC_(C) forcontrolling the constant current sources 8A to 8C, in response to thephase signal SPHASE. As a result, series-connected exciting coils 2A and2B, 2B and 2C, and 2C and 2A are consecutively energized in response tothe phase signal SPHASE, to rotate the rotor of the DC motor 1.

Generally, the motor has a predetermined relationship between the drivepower and torque (or mechanical energy). Accordingly, by controlling thedrive current, the torque generated in the motor can be freelycontrolled. In other words, when a load on the motor is varied, thetorque generated in the motor can be maintained at a predeterminedconstant value by supplying a constant current to the exciting coils. Inaddition, in the DC motor, a large start-up current may flow into thecoils for a lengthy start-up time, due to a large inertia of the rotor.This basically requires a bulky and high-cost power supply for supplyingsufficient start-up current during a long start-up time. When theconstant current drive circuit is provided, the start-up current is verylimited, enabling a reduction of the power supply. As discussed above,the constant current drive circuit 8 contributes to obtaining the aboveadvantages. Furthermore, when the constant current drive circuit isemployed for a phase-exchange-type DC motor as shown in FIG. 1, andaccordingly, may include switching power transistors, the constantcurrent drive circuit provides the phase exchange function.

Referring back to FIG. 1, in the DC motor 1, acounter-electromotive-force (emf) is induced in the exciting coils 2A to2C during the rotation of the rotor, and the amplitude of eachcounter-emf is enlarged in response to an increase in that rotation.Accordingly, a voltage of the power supply 7 is designed so that it willovercome the counter-emf at a required high rate of speed, e.g., 3600RPM, of the rotor and enable a constant current control.

The characteristics of the DC motor can be expressed by the followingformula: ##EQU1##

where, V_(M) : voltage supplied to the motor (V),

K_(e) : induced voltage constant (V),

R_(S) : speed of the rotor (RPM),

L: inductance of the series-connected coils (H),

r_(M) : resistance of the series-connected coils (Ω) and

i: current flowing through the series-connected coils.

During the start-up operation of the motor, or at a low speed operation,the speed R_(S) is almost zero or very low and the counter-emf is almostzero or very small. As a result, in spite of the provision of theconstant current drive circuit 8, a large current is still supplied tothe power transistor type switches 6A to 6C and the power transistors inthe constant current sources 8A to 8C, and accordingly, these powertransistors accumulate heat. The start-up time may be approximately 25to 35 seconds when the DC motor is used for driving a large-scalemagnetic disc system. Therefore, taking these conditions intoconsideration, high power transistors having a tolerance for a largecurrent passing therethrough and a high temperature thereat during alengthy start-up time must be provided. This results in thedisadvantages of high cost, a bulky circuit configuration, and theinstallation of expensive and bulky cooling members. In addition, theprobability of breakage of the power transistors is increased, reducingthe reliability of the DC motor drive system. Among other elements, thepower transistors of the switches 8A to 8C suffer from the latterproblem, because these transistors are used in a linear region of thecharacteristics thereof.

A strong demand for a reduction or elimination of the above problems hasarisen.

JPA No. 57-183281, published on Nov. 11, 1982, discloses a speed controlcircuit for a brushless DC motor. As shown in FIG. 4 of JPA No.57-183281, the circuit avoids the application of excess power to acurrent control power transistor 12 during the start-up of the motor byproviding a switch 25 and a resistor 23 connected to coils 13 to 15. Atthe start-up time, the resistor 23 consumes power from a power supply,and accordingly, causes a drop in the voltage supplied to the transistor12 through the coils 13 to 15 and phase exchange transistors in acurrent drive circuit 6. After the start-up, the switch 25 is energizedto bypass the resistor 23 so that a normal voltage from the power supplyis supplied to the coils 13 to 15 and the transistor 12. The aboveenergization of the switch 25 is carried out in response to a speed ofthe rotor.

This speed control circuit overcomes a part of the above problems, butsince the above switching of the voltage-changeable supply circuit isessentially a single switching, the use of the speed control circuit islimited to only a small DC motor which has a short start-up time. Inaddition, the voltage-changeable supply circuit consisting of the switchand the resistor can not fully overcome the above problems.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a DC motor drive systemwherein a start-up current can be effectively reduced.

Another object of the present invention is to provide a DC motor drivesystem enabling a simple circuit construction, the reduction ofproduction costs and an increased reliability, in addition toeffectively reducing a start-up current of a DC motor.

In a DC motor drive system including a DC motor, a unit for detecting arotation of a rotor of the motor, a power supply unit for supplying adrive power to the motor, and a current drive circuit operativelyconnected to a coil(s) of the motor and providing a constant currentpassing through the coil supplied with the drive power during a normaloperation of the motor. According to the present invention the powersupply unit receives a rotation signal from the rotational detectingunit, and sequentially increases a voltage therefrom in multi-steps inresponse to the increase of the rotation signal during a start-upoperation of the motor. As a result, a current(s) passing through thecurrent drive circuit is reduced during the start-up operation.

Preferably, the power supply unit may change the voltage therefrom in astepwise manner so that a maximum value of the current is sequentiallyincreased during the start-up.

The power supply unit may include a first power supply supplying a lowvoltage, a second power supply supplying a high voltage, a first switchcircuit including a diode connected to the first power supply andsupplying the low voltage to the coil at an initial condition, and atleast one second switch circuit including a switching element connectedto the second power supply and supplying the high voltage andreverse-biasing the diode when the second switch circuit is energizedwhen the rotation exceeds a predetermined value.

The power supply may also include a first power supply supplying avoltage having a positive polarity, a second power supply supplying avoltage having a negative polarity, a first switching circuitoperatively connected between a ground and the coil, and a secondswitching circuit operatively connected between the first power supplyand the coil. The second power supply is operatively connected to thecoil through the current drive circuit. The first switching circuit isenergized to provide a low voltage defined by ground and the negativevoltage at an initial condition so that a current defined by the lowvoltage flows between ground and the second power supply through thecoil and the current drive circuit. The second switching circuit isenergized to provide a high voltage defined by the positive voltage andthe negative voltage when the rotation exceeds a predetermined value, sothat a current defined by the high voltage flows between the first powersupply and the second power supply through the coil and the currentdrive circuit.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of prior art DC motor drive system:

FIG. 2 is a circuit diagram of a first embodiment of a DC motor drivesystem in accordance with the present invention;

FIGS. 3a to 3d are waveforms of Hall sensor outputs in FIG. 2;

FIG. 4 is a graph of the operation of the DC motor drive system in FIG.2;

FIGS. 5a to 5c are waveform diagrams of voltages between coils in the DCmotor shown in FIG. 2;

FIGS. 6a to 6c are waveform diagrams of currents through the coils inthe DC motor shown in FIG. 2;

FIGS. 7 and 8 are circuit diagrams of second and third embodiments of aDC motor drive system in accordance with the present invention;

FIG. 9 is a graph of the operation of a DC motor drive system inaccordance with the present invention;

FIGS. 10 and 11 are detailed circuit diagrams of the second and thirdembodiments shown in FIGS. 7 and 8;

FIGS. 12a to 12j are timing charts of the operation of a speed detectorshown in FIG. 11;

FIGS. 13a to 13c are circuit diagrams of a voltage-changeable powersupply unit shown in FIG. 10;

FIGS. 14a to 14e are circuit diagrams of another type ofvoltage-changeable power supply unit shown in FIG. 10;

FIGS. 15a and 15b are power MOS-FETs applicable to the circuits shown inthe embodiments;

FIGS. 16a and 16b are graphs of the characteristics of a power bipolartransistor and a power MOS-FET used in the circuits of the embodiments;

FIG. 17 is a circuit diagram of still another embodiment of a DC motordrive system in accordance with the present invention;

FIGS. 18a to 18i are timing charts representing the operation of the DCmotor drive system shown in FIG. 17; and

FIG. 19 is a block diagram of yet another embodiment of a DC motor drivesystem in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of a DC motor drive system of the present invention,corresponding to the DC motor drive system shown in FIG. 1, will bedescribed with reference to FIG. 2.

In FIG. 2, the DC motor drive system includes a speed detector 9, athreshold circuit 10 and a voltage change circuit 11, in addition to thebipolar brushless three phase DC motor 1 including the exciting coils 2Ato 2C, the rotor and the Hall sensors 3A to 3C, the signal synthesizingcircuit 4, the timing control circuit 5, the phase exchange switchingcircuit 6 having the power transistor type switches 6A to 6C, and thecurrent drive circuit 8 having the constant current sources 8A to 8C, asshown in FIG. 1.

The Hall sensors 3A to 3C output the sensed signals SHG_(A) to SHG_(C)as shown in FIGS. 3b to 3d in response to a geometrical rotational angleof the rotor, as shown in FIG. 3a. The sensed signals SHG_(A) to SHG_(C)are synthesized at the signal synthesizing circuit 4. The synthesizingcircuit 4 generates, on one hand, the phase signal SPHASE supplied tothe timing control circuit 5, and on other hand, a source signal fordetecting the rotational speed of the rotor at the speed detector 9.

The speed detector 9 detects the speed of the rotor on the basis of thesource signal from the signal synthesizing circuit 4. The speed detector9 can be realized as either a digital type speed detector or an analogtype speed detector. The former may include a counter which counts theHall sensed signal(s) SHG_(A) to SHG_(C) through the signal synthesizingcircuit 4 for a predetermined period and outputs a counted value as thespeed signal SPD. The latter may include a signal integrator whichintegrates the Hall sensed signal(s) through the signal synthesizingcircuit 4 for a predetermined period and outputs an integrated voltageas the speed signal SPD.

The threshold circuit 10 receives the speed signal SPD from the speeddetector 9 and outputs speed discrimination signals SDS₁ and SDS₂ inresponse to threshold values STH₁ and STH₂. The threshold values STH₁and STH₂ correspond to speed SPD₁ and SPD₂ indicated on an abscissa inFIG. 4 and define three speed region steps; a low speed region, a middlespeed region, and a high speed region. When the speed signal SPD is acounted value, the threshold circuit 10 may be realized by two digitalcomparators for outputting the speed discrimination signals SDS₁ andSDS₂ , respectively. When the speed signal SPD is an analog voltage, thethreshold circuit 10 may be realized by two analog comparators each ofwhich may include an IC amplifier.

The voltage changeable circuit 11 receives a constant voltage from thepower supply 7 and changes a voltage output therefrom in response to thespeed discrimination signals SDS₁ and SDS₂. The voltage from the voltagechangeable circuit 11 is known by a curve CV1 in FIG. 4. When both speeddiscrimination signals SDS₁ and SDS₂ are low level, i.e., when the rotorrotates at the low speed region in FIG. 4, the lowest voltage V₀ isselected and supplied to the exciting coils 2A to 2C and the constantcurrent drive circuit 8 through the phase exchange switching circuit 6.When only the speed discrimination signal SDS₁ is high level, i.e., whenthe rotor is in the middle speed region, a middle voltage V₁ is output.Further, when the speed discrimination signal SDS₂ is high level, i.e.,when the rotor is the high speed region, a rated voltage V₂ is output.

In FIG. 4, an abscissa represents the rotor speed, wherein curve CV1indicates the voltage output from the voltage changeable circuit 11.Curve CV2 indicates the counter-emf generated in the series-connectedexciting coils of the DC motor 1. Curve CV3 indicates the currentpassing through the series-connected exciting coils. A dotted line CV4indicates a constant rated voltage in the prior art, and another dottedline CV5 indicates the current passing through the series-connectedexciting coils when the voltage shown by the line CV4 is supplied to theexciting coils. A first ordinate represents a current for curves CV3 andCV5, and a second ordinate represents a voltage for curves CV1, CV2 andCV4.

When a certain value of the voltage is supplied to the phase exchangeswitching circuit 6, a terminal voltage V_(A) -V_(B) between outputterminals of the power transistors switches 6A and 6B, i.e., betweenterminals T1A and T1B of the coils 2A and 2B, a terminal voltage V_(B)-V_(C), and a terminal voltage V_(C) -V_(A), are illustrated as shown inFIGS. 5a to 5c. Accordingly, a current I_(2A) flowing into the coils 2Aand 2B through the terminal T1A by energizing the power transistorswitch 6A and the transistor(s) in the constant current source 8B inresponse to the timing signal ST_(A) and the control signal SC_(B) fromthe timing control circuit 5, respectively, i.e., a current passingthrough the constant current source 8B is shown in FIG. 6a. Similarly, acurrent I_(2B) flowing into the coil 2B and a current I_(2C) flowinginto the coil 2C are shown in FIGS. 6b and 6c. Curves CV3 and CV5indicate the amplitude of the current.

In FIG. 4, the counter-emf, as shown by curve CV2, at the coils isincreased in response to the increase in the rotor speed. The currentflowing into the series-connected coils, i.d., the current passingthrough the current source, is determined by the relationship expressedby formula (1). As described in the prior art, the voltage supplied tothe phase exchange switching circuit 6 is constant rated voltage V₂ asshown by the dotted line CV4. The current therefor is shown by thedotted line CV5. As a result, an initial current at a zero speed isI'_(MAX) in FIG. 4, which may be several tens of times greater than acurrent I_(R) at a rated speed although the current drive circuit isprovided. Note that an ordinate of the current is shown as a logarithmicscale. In the first prior art, the high initial current I'_(MAX) causesa variety of problems as discussed above.

According to the above multiple change of the voltage supplied to thephase exchange switching circuit 6 as shown by curve CV1 in theembodiment of FIG. 2, an initial current defined by the lowest voltageV₀ is greatly reduced to I_(MAX) , as shown by curve CV3. The current isalso reduced to I_(MIN) in response to the increase of the rotationspeed, i.e., the increase of the counter-emf. As the voltage is revisedto V₁ at the speed SPD₁ , the current is increase to a certain valuesmaller than I_(MAX). The above phenomenon in the low speed region isrepeated in the middle speed region and the high speed region. When therotor speed reaches a rated speed, the current reaches a rated valuei_(R) defined by the formula (1) of the rated voltage V₂ as the voltageV_(M) in the formula (1).

In the prior art disclosed in JPA No. 57-18321, a similar effect can beobtained. However, the prior art may not obtain a sufficiently lowinitial current because of a single voltage switching. Morespecifically, the rated current I_(R) at the rated speed must be kept ata predetermined value, and the division of the speed range is rough. Asa result, the current may be illustrated by curve CV6. An initialcurrent may be I"_(MAX) lower than I'_(MAX) , but much higher thanI_(MAX).

Since the current is reduced throughout all operating conditions in theembodiment, low rated switching elements in the phase exchange switchingcircuit 6 and the current drive circuit 8 can be used with a highreliability, and accordingly, the problems discussed above are overcome.

As shown in FIG. 7, the voltage changeable circuit 11 shown in FIG. 2 isrealized by a circuit including series-connectable resistors 111 and 112and power transistor switches 113 and 114. The power supply 7 outputsthe rated voltage V₂. When the rotation speed is in the low speedregion, the switches 113 and 114 are deenergized to connect theresistors 111 and 112 in series so that the lowest voltages V₀ is outputtherefrom. When the rotation speed is in the middle speed region, theswitch 113 is energized to bypass the resistor 111 so that the middlevoltage V₁ is output. When the rotation speed is in the high speedregion, both switches 113 and 114 are energized to bypass both resistors111 and 112, so that the rated voltage V₂ is output.

A modification of the system shown in FIG. 7 can be made as shown inFIG. 8. A power supply 7a provides voltages of V₀ , V₁ , and V₂. Avoltage changeable circuit 11a includes three parallel-connected powertransistor switches 115 to 117, and a threshold circuit 10a providesdiscrimination signals SDS'₁ , SDS'₂ , and SDS'₃ for energizing theswitches 115 to 117, respectively.

A multiple-step voltage switching greater than three, as shown in FIGS.2, 7, and 8, is preferable.

In addition, the voltage switching circuits shown in FIGS. 7 and 8 canbe combined to realize a multiple-step voltage switching.

As discussed above, the initial (or start-up) current should be as smallas possible, from the viewpoint of reducing the load on the powertransistors. This means that the low voltage V₀ must be as low aspossible. On the other hand, the rated voltage supplied to the DC motorin the high speed region should be as high as possible, because thecurrent (I) flowing in the coils of the DC motor becomes low, andaccordingly, a loss in proportion to I₂ in the DC motor is reduced. FIG.9 is a graph of the above features, wherein curve CV11 represents thevoltage supplied to the DC motor and corresponds to curve CV1 shown inFIG. 4. The start-up voltage V₀ may be equal to that of FIG. 4, but therated voltage V₃ is higher than the rated voltage V₂ of FIG. 4. Curve 12represents the current flowing through the coils and corresponds tocurve CV3 in FIG. 4. Note, the start-up current I_(MX) may be equal toI_(MAX) in FIG. 4, but the rated current I'_(R) is lower than I_(R) inFIG. 4.

From the above viewpoint, a multiple step voltage switching ispreferable.

The resistors 111 and 112, the power supply 7 in FIG. 7, and the powersupply 7a in FIG. 8 can be designed to meet the features discussedabove.

Other embodiments of the DC motor drive system of the present inventionwill be more concretely described with reference to the drawings.

FIG. 10 is a circuit diagram of a DC motor drive system, but illustratedmore specifically than the DC motor drive system shown in FIGS. 2, 7,and 8.

In FIG. 10, the same references as those used in FIGS. 2, 7 and 8 areused for the same components. Reference 20 denotes a voltage-changeablesupply unit corresponding to the circuit combined with the power supply7 or 7a and the voltage changable circuit 11 or 11a in FIGS. 2, 7, and8. The signal synthesizing circuit 4 is omitted.

The timing control circuit 5 includes a decoder 51 receiving the sensedsignals SHG_(A) to SHG_(C) , a driver gate circuit 52 for outputting thephase exchange timing signals ST_(A) to ST_(C), and a driver gatecircuit 53 for outputting the control signals SC_(A) to SC_(C). Thedecoder 51 generates the timing signals ST_(A) to ST_(C) and the controlsignals SC_(A) to SC_(C), in response to the sensed signals SHG_(A) toSHG_(C) indicating the rotational angle of the rotor.

To simplify the circuit diagram, a single power transistor type switch6C in the power exchange switch circuit 6 and a single constant currentsource 8C in the constant current drive circuit 8, both connected to thecoil 2C, are shown. The power transistor type switch 6C includes atransistor Q61, a transistor Q62 functioning as an operational amplifierand a Darlington circuit 61 including power transistors Q63 and Q64. Thetransistor Q61 is turned ON in response to the timing signal ST_(C), toplace the Darlington circuit 61 in an ON state through the transistorQ62, and to supply a voltage from the voltage-changeable supply unit 20.The constant current source 8C includes three parallel-connected powertransistors Q82 to Q84, and a transistor Q81 functioning as anoperational amplifier. The transistor Q81 is turned ON in response tothe control signal SC_(C) to energize the power transistors Q82 to Q84,so that constant currents flow therethrough during normal operation ofthe motor. The constant current drive circuit 8 includes a commoncurrent limiter 80 including a transistor Q80 and a comparator CMP80.The limiter 80 limits the currents passing through the power transistorsQ82 to Q84 in the current source 8C to a limit value I_(LMT) supplied tothe comparator CMP80.

FIG. 11 is a circuit diagram of the speed detector 9 and the thresholdcircuit 10. The speed detector 9 includes an AND gate AND91 receivingthe Hall signals SHG_(A) and SHG_(C), OR gates OR91 and OR92,seriesconnected flip-flops (FFs) FF91 to FF93, an AND gate AND92, and FFFF94, and an inverter INV91. A clock CLK is supplied to the delay-typeFFs FF92 to FF94. Figures 12a to 12j are timing charts of the speeddetector 9, and FIGS. 12e to 12h are views of signals at nodes N1 to N4.The speed detector 9 outputs signals *RST1 and RST2 indicating therotation speed SPD.

The threshold circuit 10 includes four OR gates, two FFs FF101 andFF102, and output transistors Q101 and Q102. The threshold circuit 10receives the speed signals *RST1 and RST2 and outputs the discriminationsignals SDS₁ and SDS₂ to the voltage-changeable supply unit 20 inaccordance with the threshold signals STH₁ and STH₂.

A variety of embodiments of the voltage-changeable supply unit 20 shownin FIG. 10 will be described with reference to FIGS. 13a to 13c, andFIGS. 14a to 14e.

In FIG. 13a, a voltage-changeable supply unit 20a includes a first powersupply 201 supplying a constant voltage VC₁ corresponding to the lowestvoltage V₀ shown in FIGS. 4 and 9, a second first power supply 202supplying a constant voltage VC₂ corresponding to the rated voltage V₂shown in FIG. 4, a diode 203, and a switch 204. The voltage supply unit20a is connected to the phase exchange switching circuit 6. In aninitial condition, the switch 204 is turned OFF and the voltage VC₁ fromthe power supply 201 is supplied to the phase exchange switching circuit6. When the rotation speed exceeds a predetermined value, thediscrimination signal SDS is output from the threshold circuit 10,turning the switch 204 ON. As a result, the doide 203 is reversebiasedand automatically turned OFF because the voltage VC₂ is higher than thevoltage VC₁, and the voltage VC₂ is supplied to the phase exchangeswitching circuit 6 through the switch 204. Compared with prior artreference JPA No. 57-183281, a power loss at the resistor in JPA No.57-183281 is eliminated. In addition, the low cost and high reliabilitydoide 203 reduces the size of the switch 115 shown in FIG. 8. Theswitches 204, and 115 to 117 shown in FIG. 8 must be constructed using apower transistor(s) and a relatively complex circuit, as shown by theswitch circuit 6C including the Darlington connected power transistorsQ63 and Q64, because a high voltage is supplied thereto and a largecurrent is passed therethrough. Accordingly, the reduction of the sizeof the switch by providing the diode 203 provides a simple circuitconstruction and reduces the cost thereof.

In FIG. 13b, a voltage-changeable supply unit 20b includes a resistor205 in addition to the voltage supply unit 20a shown in FIG. 13a. Thepower supply 201' also differs from the power supply 201 shown in FIG.13a. Generally, a power supply is standardized to supply 12 VDC, 24 VDC,48 VDC, etc. When the lowest voltage V₀ is 9 VDC, the power supply 201shown in FIG. 13a is provided as a special power supply of 8 VDC. InFIG. 13b, the resistor 205 is designed to reduce the voltage of 12 VDCto 8 VDC. Accordingly, a standard, inexpensive power supply 201' havinga voltage of 12 VDC is applicable.

A voltage-changeable power supply unit 20c in FIG. 13c is a modificationof the power supply unit 20b in FIG. 13b. The power supply 201', thediode 203 and the resistor 205 provide the voltage V₀ in the low speedregion. The power supply unit 202 provides the voltage V₂, for example,48 VDC, wherein a resistor 206 is designed to reduce the voltage V₂ tothe voltage V₁ shown in FIG. 4. The power supply 202, the resistor 206,and a switch 207 provide the voltage V₁ in the middle speed region. Thepower supply 202 and the switch 204 provide the voltage V₂ in the highspeed region. The power supply unit 20c functions in the same way as thecircuits 7a and 11a, but one switch and one power supply are omitted incomparision with those circuits.

The power supply unit 20c is able to supply many more voltage levels.This is discussed in conjunction with the voltage change as shown bycurve CV1 in FIG. 4. The voltage supply units may be designed to providethe voltage change as shown by curve CV11 in FIG. 9.

Another type of voltage-changeable power supply unit 20 will be desribedwith reference to FIGS. 14a to 14e. The main feature of this type ofvoltage supply unit is the provision of a positive voltage power supply211 and a negative voltage power supply 212 between the phase exchangeswitching circuit 6, the DC motor 1, and the constant current drivecircuit 8. As described above, the voltage at the high speed operationshould be as high as possible, to minimize power loss in the motor.However, a high voltage power supply is subject to strict requirements,such as safety regulations and very good insulation, and may beexpensive. The positive voltage power supply 211 and the negativevoltage power supply 212 share a high voltage and provide the highvoltage therebetween. For example, when the high voltage is 48 VDC, thepositive voltage power supply 211 provides +24 VDC and the negativevoltage power supply 212 provides -24 VDC.

A voltage supply unit in FIG. 14a provides a low voltage between theground and the negative voltage power supply 212 to the motor 1 throughthe phase exchange switching circuit 6 and the constant current drivecircuit 8, when a switch 214 is energized by the discrimination signalSDS₁ in the low speed region. The voltage supply unit provides a highvoltage between the positive voltage power supply 211 and the negativevoltage power supply 212 to the motor 1, when a switch 213 is energizedand the switch 214 is deenergized in the high speed region.

A voltage supply unit shown in FIG. 14b provides three voltages: a lowvoltage between the ground and the power supply 212, a high voltagebetween the power supplies 211 and 212, and a middle voltage lower thanthe high voltage by a voltage drop at a resistor 215. The switches 214,216 and 213 are consecutively energized in response to the rotationalspeed of the motor 1.

A voltage supply unit shown in FIG. 14c includes a diode 217 and aresistor 218 instead of the switch 214 in FIG. 14a. The operationalfunctions of the diode 217, the resistor 218, and the switch 213 aresimilar to those shown in FIG. 13b.

A voltage supply unit shown in FIG. 14d is combined with the voltagesupply units shown in FIGS. 14b and 14c. This voltage supply unit alsosupplies three voltages.

The voltage supply unit shown in FIG. 14e is a modification of thevoltage supply unit in FIG. 14d. In the middle speed region, the switch216 is energized to reverse-bias the diode 217 and to series-connect aresistor 219 and the resistor 218. The resistor 218 is used not only inthe low speed region but also in the middle speed region. The resistor219 can be smaller than the resistor 215 in FIG. 14d, and thus thevoltage supply unit in FIG. 14e is more economical than the voltagesupply unit in FIG. 14d.

In the above embodiments, switching elements as the switches for thepower transistor type switch 6c in FIG. 10, the current source 8c inFIG. 10, the switch 204 in FIG. 13a, and the switches 213 and 214 inFIG. 14a are power transistors. As shown in FIG. 10, the powertransistor is generally used to ground the emitter and the output fromthe collector.

The switching elements can be replaced by other switching elements, forexample, power MOS-FETS. Generally, a MOS-FET is used to ground a source(S) as shown in FIGS. 15a and 15b. The MOS-FET is FIG. 15a can be usedas a phase exchange switch instead of the phase exchange switchingcircuit 6C in FIG. 10. The MOS-FET in FIG. 15b can be used as a constantcurrent source instead of the constant current source 8c in FIG. 10.

A recent technological advance in the MOS-FET field enables practicaluse of a power MOS-FET having a low ON-resistance and operable by lowgate voltage, such as a TTL voltage level. As an impedance of a gate ofthe power MOS-FET is very high, a gate current is very low, resulting ina low power consumption, and a simple circuit construction.

In addition, the MOS-FET is not subjected to a second break downlimitation in a bipolar transistor. FIG. 16a is a graph illustrating thecharacteristics of the bipolar transistor. The performance of thebipolar transistor is limited by a current limit shown by a dotted lineLMT_(C), a limit of a collector emitter voltage V_(CE) shown by a dottedline LMT_(V), and a wattage limit shown by a dotted line LMT_(W). Thebipolar transistor is further limited by the second break downlimitation of a junction shown by a solid line LMT_(SBD), to preventthermal dissipation. As a result, an effective region of the bipolartransistor for actual use is limited to that shown as a shaded region.The second break down limitation greatly depends on the temperaturethereat. If the temperature rises, the second break down limitation isincreased as shown by a line LMT'_(SBD), further narowing the effectiveuse region. Conversely, the MOS-FET is not subjected to the above secondbreak down limitation as shown in FIG. 16b, because the MOS-FET does nothave a junction. In FIG. 16b, an abscissa represents a drain sourcevoltage and an ordinate represents a drain current. Lines LT_(C),LT_(W), and LT_(V) indicate a current limitation, a wattage limitation,and a voltage limitation. A shaded portion indicates an effective useregion.

FIG. 17 is a diagram of another type of a DC motor drive system.

In FIG. 17, a DC motor 1a is a unipolar type brushless motor having aneutral line 2D, exciting coils 2A to 2C corresponding to those in FIG.10, and Hall sensors 3A' to 3C'. The signal synthesizing circuit 4receives Hall sensed signals SHG'_(A) to SHG'_(C) and outputsinverted-signals as a phase signal through inverters INV41 to INV43. Thetiming control circuit 5a includes inverters INV51 to INV53, AND gatesAND51 to AND53, and drivers DRV51 to DRV53. The timing control circuit5a outputs control signals SC_(A) to SC_(C) to a constant current drivecircuit wherein only one current source 8c' and a current limiter 80'are illustrated. The current source 8c' is similar to the current source8c shown in FIG. 10. The current source 8c' includes a transistor Q86',a transistor Q81' functioning as an operational amplifier, and fourparallel-connected power amplifiers Q82; to Q85'.

The speed detector 9, the threshold circuit 10, and the voltage supplyunit 20 are substantially the same as to those described above.

Note that the phase exchange switching circuit 6 in FIG. 10 is notnecessary.

FIGS. 18a to 18i are timing charts illustrating the operation of the DCmotor drive system in FIG. 17. FIGS. 18a to 18c are waveform diagrams ofthe Hall sensor's outputs SHG'_(C) to SHG'_(A), FIGS. 18d to 18f arewaveform diagrams of voltages between the neutral line, and the coils2C, 2B and 2A, respectively, and FIGS. 18g to 18i are currents I_(C) toI_(A) flowing into the coils 2C, 2B and 2C. During the shaded time, thecurrents flow into the coils through the constant current drivercircuits 8a', 8b' (not shown), and 8c'.

The power MOS-FETS also can be used in the circuit shown in FIG. 17.

In the above embodiments, the DC motor drive system for driving theHall-type motor is a brushless DC motor. The DC motor drive system ofthe present invention obviously can be applied to drive other types ofbrushless DC motors in MR devices, magnetic saturation devices,photo-interruptors, etc.

The DC motor drive system can be applied not only to brushless DC motorsbut also to brush-type DC motors deiven by a constant current drivecircuit and requiring a reduction of the start-up current.

FIG. 19 is a block diagram of a brush type DC motor drive system. Thedrive system includes a voltage changeable power supply unit 20', abrush type DC motor 1', a speed sensor 3', such as a tacho-generatormechanically connected to a rotor shaft of the DC motor 1', and aconstant current drive circuit 81. Here, phase exchange switching is notnecessary. The voltage changeable supply unit 20; receives a speedsignal from the speed sensor 3' and outputs a variety of voltage stepsin response to the rotor speed. The current drive circuit 8' can berealized by the circuits discussed above.

The DC motor drive system of the present invention is applicable to avariety of systems using DC motors, such as a magnetic disc drivesystem. Preferably, the DC motor drive system is applied to a DC motordrive having a constant current drive circuit and maintaining a constanttorque regardless of a load change on the DC motor.

We claim:
 1. A direct-current motor drive system comprising:a DC motor;means, connected to said DC motor, for detecting a rotation of a rotorof said DC motor and outputting a rotational signal; power supply means,connected to said DC motor, for supplying drive power to said DC motor;current drive means, operatively connected to a coil of said DC motorand providing a constant current passing through said coil supplied withthe drive power during a normal operatin of said DC motor, said powersupply means receiving the rotational signal from said rotationaldetecting means, and sequentially increasing a voltage therefrom inmulti-steps in response to the increase of the rotational signal duringa start-up operation of said DC motor, a current defined by said voltageand passing through said current drive means being reduced to a valueless than a predetermined value.
 2. A DC motor drive system according toclaim 1, wherein said power supply means includes means for stepwiselychanging said voltage in response to the increase of the rotationalsignal so that a maximim value of said current defined by said voltagein each step and passing through said current drive means issequentially increased during said start-up operation.
 3. A DC motordrive system according to claim 2, wherein said current drive meanscomprises:at least one current drive circuit, each said current drivecircuit including: at least one power switching element operativelyconnected to said coil and providing the constant current passingthrough said coil during said normal operation, the current passingthrough said power switching element being increased in response to theincrease of said rotational signal during said start-up operation.
 4. ADC motor drive system according to claim 3, wherein each said currentdrive circuit further comprises an operational amplifier for drivingsaid power switching element.
 5. A DC motor drive system according toclaim 4, wherein said power supply means comprises at least twoswitching circuits connected in parallel, each of said switchingcircuits being energized to provide a predetermined rotation rangedifferent from a rotation range for the other switching circuit, byproviding a voltage different from another voltage from said otherswitching circuit.
 6. A DC motor drive system according to claim 5,wherein each said switching circuit comprises a power switching elementoperating between a fully turned-ON state and fully turned-OFF state. 7.A DC motor drive system according to claim 6, wherein said DC motor is abrushless-type DC motor, including a plurality of exciting coils,whereinsaid current drive circuits are respectively operatively connected tosaid exciting coils, and wherein said DC motor drive system furthercomprises timing control means, connected to said rotational detectingmeans, for receiving the rotational signal, determining a rotationalphase and consecutively generating a plurality of control signals tosaid respective current drive circuits.
 8. A DC motor drive systemaccording to claim 6, wherein said DC motor is a brushless, phaseexchange type DC motor including a plurality of exciting coils,whereinsaid current drive circuits are respectively operatively connected tosaid exciting coils, and wherein said DC motor drive system furthercomprises: phase exchange means including a plurality of switchingcircuits, each operatively connected to said power supply means, saidrespective current drive circuits and said respective exciting coilsproviding a rotational force to said rotor when energized; and timingcontrol means, connected to said rotational detecting means, saidswitching circuits and said current drive circuits, for receiving therotational signal, determining a rotational phase, and consecutivelyoutputting a plurality of timing signals to said respective switchingcircuits in said phase exchange means and a plurality of control signalsto said respective current drive circuits.
 9. A DC motor drive systemaccording to claim 8, wherein each said switching circuit in said phaseexchange means comprises a power switching element which operatesbetween a fully turned-ON state and a fully turned-OFF state.
 10. A DCmotor drive system according to claim 3, wherein each of said powerswitching elements comprises a power bipolar-transistor.
 11. A DC motordrive system according to claim 3, wherein each of said power switchingelements comprises a power MOS-FET.
 12. A system for driving adirect-current motor including a rotor, at least one exciting coil andmeans for detecting a rotation of the rotor, comprising:power supplymeans, connected to the motor, for supplying a drive power to the motor;and current drive means, operatively connected to the coil and providinga constant current passing through the coil supplied with the drivepower during a normal operation state of the motor, said power supplymeans including: a first power supply supplying a low voltage; a secondpower supply supplying a high voltage, higher than the low voltage; afirst switch circuit including a diode, connected to said first powersupply and supplying the low voltage to the coil at an initialcondition; and at least one second switch circuit including a firstswitching element, connected to said second power supply and supplyingthe high voltage and reverse-biasing said diode when said second switchcircuit is energized when rotation of said rotor exceeds a predeterminedvalue.
 13. A system according to claim 12, wherein said second switchcircuit comprises a second switching element connected between saidsecond power supply and a phase exchange means.
 14. A system accordingto claim 13, wherein said first power supply comprises a conventionalpower supply (201') for supplying a standard voltage, andwherein saidfirst switch circuit includes a resistor connected in series to saiddiode, for supplying said low voltage to the coil from said first powersupply.
 15. A system according to claim 14, wherein said second switchcircuit further comprises at least one or more switching circuitsconnected in parallel to said first switching element, each of saidswitching circuits including a circuit having a third switching elementconnected in series with a resistor, and supplying a voltage betweensaid low voltage and said high voltage, said voltage being different foreach of said at least one or more switching circuits, and wherein thevoltages from said power supply means being such that one of saidvoltages is output in response to rotation of the motor and sequentiallyincreasing to a maximum value of the current passing through saidcurrent drive means, determined by said output voltage in response tothe increase in rotation of the motor.
 16. A system according to claim13, wherein said second switch circuit fruther comprises:at least one ormore switching circuits connected in parallel to said first switchingelement, each of said at least one or more switching circuits including:a third switching element; and a resistor, connected in series with saidthird switching element, said second switching element and said resistorsupplying a voltage between said low voltage and said high voltage, saidvoltage being different for each of said at least one or more switchingcircuits, and wherein the voltages from said power supply means beingsuch that one of said voltages is output in response to rotation of themotor and sequentially increasing to a maximum value of the currentpassing through said current drive means, determined by said outputvoltage in response to the increase in rotation of the motor.
 17. Asystem according to claim 13, wherein each of said switching elementscomprises a power bipolar transistor.
 18. A system according to claim13, wherein each of said switching elements comprises a power MOS-FET.19. A system for driving a direct current motor including a rotor, atleast one exciting coil, and means, connected to the rotor, fordetecting a rotation of the rotor, said system comprising:power supplymeans, connected to the motor, for supplying a drive power to the motor;and current drive means, operatively connected to the coil and providinga constant current passing through the coil supplied with the drivepower during a normal operation state of the motor, said power supplymeans including: a first power supply supplying a positive voltage; asecond power supply supplying a negative voltage; first switching means,operatively connected between ground and the coil, for switching betweenground and the coil; and second switching means, operatively connectedbetween said first power supply and the coil, for switching between saidfirst power supply and the coil, said second power supply operativelyconnected to the coil through said current drive means, said firstswitching means being energized at an initial condition to provide a lowvoltage defined by ground and said negative voltage of said second powersupply so that a current defined by said low voltage flows betweenground and said second power supply through the coil and said currentdrive means, and said second switching means being energized to providea high voltage defined by said positive voltage of said first powersupply when rotation of the motor exceeds a predetermined value so thatthe current defined by said high voltage flows between said first powersupply and said second power supply through the coil and said currentdrive means.
 20. A system acording to claim 19, wherein said firstswitching means comprises a first switching element, being energized atsaid initial condition, so that the current, defined by a voltagebetween ground and said negative voltage of said second power supply,flows through the coil and said current drive means, and beingdeenergized when the rotation of the motor exceeds the predeterminedvalue.
 21. A system according to claim 20, wherein said second switchingmeans comprises a second switching element being energized when therotation of the motor exceeds the predetermined value.
 22. A systemaccording to claim 21, wherein said second switching means furthercomprises at least one or more switching circuits connected in parallelto said second switching element, each of said switching circuitsincluding:a third switching element; and a resistor, connected in serieswith said third switching element, supplying a voltage between said lowvoltage and said high voltage, said voltage being different for each ofsaid at least one or more switching circuits, wherein the voltages fromsaid power supply means being defined such that one of said voltages isoutput in response to rotation of the motor and sequentially increasingto a maximum value of the current passing through the coil and saidcurrent drive means, in response to the increase in rotation of themotor.
 23. A system according to claim 22, wherein said first switchingmeans comprises a diode having an anode operatively connected to ground.24. A system according to claim 23, wherein said second power supplycomprises a conventional power supply for supplying a standard voltage,andwherein a first one of said at least one or more switching circuitsincludes a resistor connected in series with said diode to supply saidlow voltage to the coil and to said current drive means from saidstandard voltage.
 25. A system according to claim 21, wherein each ofsaid switching elements comprises a power bipolartransistor.
 26. Asystem according to claim 21, wherein each of said switching elementscomprises a power MOS-FET.